i just bought this game like 3 days ago and i put the disc in and installed it and everything and when i open steam it says pre load complete or something and i cannot play it or register my cd key it only says to purchase the game online.

You need to activate your cd-key via steam, just klick the activate your product and insert your cd-key there when asked. If it won't work, contact steam support. You can also try to unistall steam and sin, reinstall steam and then just activate the sin cd-key, no need to install that retail cd, since it downloads it from steam with latest patches. Hope that helps.